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XI. On the action of crystallized surfaces upon light. By David 
Brewster, LL. D. F. R. S. Lond. and Edin. In a letter 
addressed to the Right Hon. Sir Joseph Banks, Bart. G. C. B . 
P. R. S. &c. &c. &c. 
Read February 25, 1819. 
My dear Sir, 
It has been remarked by Malus, in his Theory of Double 
Refraction," that the action which the first surface of Iceland 
“ spar exercises upon light, is independent of the position of its 
“ principal section ; — that its reflecting power extends beyond 
“ the limits of the polarising forces of the crystal, and that as 
“ light is only polarised by penetrating the surface, the forces 
“ which produce extraordinary refraction begin to act only at 
“ this limit.” He also observes, that “ the angle of incidence 
" at which Iceland spar polarises light by partial reflection, is 
“ 56° 30' ; that it then comports itself like a common transpa- 
“ rent body ; and that whatever be the angle comprehended 
“ between the plane of incidence and the principal section of 
“ the crystal, the ray reflected by the first surface is always 
“ polarised in the same manner.” * 
These conclusions, obtained experimentally by an author 
of such distinguished eminence, I should naturally have re- 
ceived as established truths, had I not been led, by a series 
of experiments made before the perusal of his work, to opi- 
nions of an opposite kind. My experiments indicated an 
extension of the polarising forces beyond the crystal ; and I 
* Tbtorie dt la Double Refraction, pp. 240, 241. 
MDCCCXIX. U 
